The advents of the Internet and wireless communication have changed public expectations about telecommunication services. The Internet has brought, among many benefits, the convenience and speed of electronic messaging such as electronic mail (email). At the same time, wireless communication has brought the convenience of mobility and nearly universal connectivity.
Cellular telephony, for example, has evolved from first-generation analog systems, to second-generation digital systems, then to “generation 2.5” systems that provide short-range wireless connectivity to wireline ports, and more recently to third-generation systems that promise to deliver high-bandwidth wireless connectivity. Parallel to this evolution, new messaging techniques have emerged, each appropriate for the mobile terminal technology of the time. Thus, messaging techniques have progressed from basic Short Message Service (SMS) through Enhanced Message Service (EMS), and soon on to Multimedia Message Service (MMS), where each step is richer in function but more complex and demanding than its predecessor.
However, as the capabilities of electronic messaging become more extensive, for example by enabling the attachment of various file types, for example images and other file types as well, to electronic messages, the limitations of mobile terminals that are ordinarily used for wireless communication become increasingly restrictive. These limitations result mainly from the need for small terminal size, which is inevitably accompanied by small displays and small keyboards, and from constraints on the extent to which radio spectrum, i.e., bandwidth, is available for wireless telecommunication services.
Measures have been adopted to help mitigate the limitations of wireless systems with mobile terminals. For example, sophisticated source-coding and channel-coding methods have been developed to conserve bandwidth, the Wireless Access Protocol (WAP) has been developed to reduce the complexity of providing Internet-like services on mobile terminals with small displays, and so forth.
Along the same general vein, new messaging formats and protocols are often made “backward compatible,” so that a mobile terminal not equipped to take advantage of the full range of information provided by the latest messaging technique may nevertheless convey a useable subset of that information. Backward compatibility is provided by designing a new messaging technique so that a state-of-the-art mobile terminal may take advantage of all of the information carried by a message, but also so that an old terminal may accept and convey parts of the same message, and simply discard other parts of the message that lie beyond its messaging capability. For example, if an EMS message is sent to a mobile terminal that is capable of supporting only SMS messaging, the more complex information contained within the EMS message will simply be ignored (i.e., discarded) by the mobile terminal. Thus, by exploiting the nature of backward compatibility, each mobile terminal can select a subset of a message that falls within its messaging capability.
In addition, the messaging capabilities of mobile terminals are sometimes deliberately limited in order to keep the cost of the mobile terminal low or its size small. For example, a mobile terminal may have a monochromatic display rather than a full color display, a small display rather than a large display, and capability to support only Wireless BitMap (WBMP) images rather than images that are richer in content. In this case, backward compatibility enables a service provider to transmit all messages according to the newest messaging technique, and have confidence that a small or low-cost terminal deliberately provided with limited function, as well as an older mobile terminal, can still convey the gist of the message.
Unfortunately, transmitting a full message and relying on a mobile terminal to select a subset of the full message while discarding the rest consumes bandwidth unproductively. This is a significant shortcoming of reliance on backward compatibility, as bandwidth is an expensive resource whose availability fundamentally constrains the capacity of wireless communication systems. Such a waste of bandwidth may therefore result in a disincentive for service providers to adopt the latest messaging techniques, as subscribers with old or limited-function mobile terminals cannot rationally be expected to bear the cost of wasted bandwidth which provides no apparent benefit.
Consequently, there is a need for a way to provide inter-generational compatibility for messaging systems that does not waste bandwidth and yet enables a new messaging system to support mobile terminals that are either older or deliberately simpler than state-of-the-art terminals that are fully compatible with the new messaging system.